Cyclopropane stereospecific opening

Among the many interconversions which occur readily in related cyclobutane, cyclopropane and open-chain frameworks, the stereospecific rearrangement of a-halo- or a-tosyloxycy-clobutanones into cyclopropanecarboxylic acid derivatives, has been shown to follow the semi-benzilic rearrangement mechanism, i. e. addition of nucleophiles to the carbonyl carbon atom and concerted displacement of halide (or tosyloxy) ions with 1,2-migration of the Ca — C carbonyl bond (see Section 4.1.2.2.5.). °... 

Johnson s stereospecific opening of donor-acceptor substituted cyclopropanes [9]... 

The ring-opening of the cyclopropane nitrosourea 233 with silver trifiate followed by stereospecific [4 + 2] cycloaddition yields 234 [129]. (Scheme 93) Oxovanadium(V) compounds, VO(OR)X2, are revealed to be Lewis acids with one-electron oxidation capability. These properties permit versatile oxidative transformations of carbonyl and organosilicon compounds as exemplified by ring-opening oxygenation of cyclic ketones [130], dehydrogenative aroma-tization of 2-eyclohexen-l-ones [131], allylic oxidation of oc,/ -unsaturated carbonyl compounds [132], decarboxylative oxidation of a-amino acids [133], oxidative desilylation of silyl enol ethers [134], allylic silanes, and benzylic silanes [135]. 

The intramolecular nucleophilic opening of diactivated cyclopropanes by amino functions has been used by Danishefsky to develop new stereospecific routes to N-heterocycles. This intramolecular ring-opening can occur in a spiro or in a fused mode (equation 199). It is shown that the spiro mode, leading to five- or six-membered rings. 

Unlike intermolecular cyclopropanation reactions, the intramolecular version is stereo-specific with respect to the configuration of both, the C-C double bond and (due to steric constraints) the carbenoid center. This fact can be used for the directed synthesis of cis-1,2-disubstituted, all-ci. -l,2,3-trisubstituted or d. , ra . -l,2,3-trisubstituted cyclopropanes. For example, a-diazo esters (a-diazocarboxamides) with an unsaturated ester (amide) residue yield bicyclic lactones (lactams) stereospecifically which can be ring opened to give the monocyclic cyclopropanes mentioned with defined stereochemistry. Some possibilities are illustrated by the examples, 20 -+ 22 -> 23, and 24 25. ... 

Several other examples of uncomplexed double-bond cyclopropanation within tricarbonyliron complexes and decomplexation without ring opening have been reported. These include alternative stereospecific pathways to hemicaronic aldehydes. 

Cyclopropane formation was also observed as a side reaction (1 9 up to 1 1) in the palladium-catalyzed coupling of ketene alkyl silyl acetals with open-chain and cyclic allyl acetates. The reaction is interpreted as proceeding via nucleophilic central attack of a jr-allyl intermediate. Although cyclopropane formation proceeds only with low yields, a highly stereospecific pathway was observed with substituted 3-cyclohexenyl acetates. ... 

The stereochemical implications of the reductive opening of the cyclopropane ring in polycycles has been studied. The three-membered ring in the isomeric 3a,5-cyclo-5a- (24) and 3, 5-cyclo-5)S-cholestan-6-one (25) was opened differently. In the latter case, the cyclopropylmethylene group became a methyl group. The reduction of 4,5-methano-3-oxosteroids was shown to be stereospecific with respect to the orientation of the methyl group formed. ... 

An interesting reaction with the simultaneous opening of a cyclopropane and a cyclobutane ring was observed when sodium methoxide was added to dimethyl 2-(l-methyl-2-oxo-cyclobutyl)cyclopropane-l,l-dicarboxylate (19).The formation of the alkenes 20 by the fragmentation process occurred in high yield and stereospecificity. 

---